One of the most important, most misunderstood and most argued about concepts in dentistry is that of "centric relation" (CR). Spear Education describes CR as the position of the lower jaw when the heads of the condyles (the tops of the lower jaw bones) are at their most forward and upward position where they meet the temporal bone. A study published in The Journal of Prosthetic Dentistry demonstrated that most dentists agree that CR is a physiologic "spatial relationship" between the lower and upper jaws that is independent of tooth contact and repeatable. Essentially, it's the position where the lower jaw joint meets the undersurface of the skull, slightly in front of the ears.
Centric Relation and the Jaw
To understand why the concept of CR is so important, you should first consider the jaw itself. The jaw is unique in the human body for three reasons.
First, it functions bilaterally. For example, you can raise one arm at a time, but you can't open one side of your jaw (or mouth) at a time.
The jaw can, however, move in different directions. Unlike a knee or elbow that can only rotate, the jaw not only rotates but also "translates." This is what happens when you protrude or move your lower jaw from side to side, such as when you chew.
Lastly, the jaw is the only joint in the body that has teeth attached to the other end of it — and how those teeth meet when you close your jaw can determine how the lower jaw relates to the skull.
The concept of CR describes the only repeatable position of the lower jaw in relation to the skull. Being repeatable means that the lower jaw can return to the same position relating to the upper jaw. This isn't something that the average person would normally feel or be aware of. You don't really think of this when you speak or chew; it just occurs naturally.
Why Is Centric Relation Important?
Historically, the concept of CR emerged to describe a position from which dental professionals could construct dentures, according to a report in the Compendium of Continuing Education in Dentistry. Therefore, it became an important consideration in orthodontia, restorative dentistry, full-mouth reconstruction and orthognathic surgery. Further, dental professionals may want to determine if the patient's jaw is functioning from a starting position of CR when attempting to treat jaw conditions.
However, while CR has been associated with diagnosing temporomandibular joint problems, there is inconclusive evidence regarding the exact relationship between the two, according to a review in Acta Odontologica Scandinavica. Complicating the question further, some dental professionals have argued that CR really refers to a range of positions rather than a single one.
Just as you wouldn't want to design or build a skyscraper on a bed of sand, you need a solid foundation to reconstruct a bite. That solid foundation is CR. By using CR as a starting point, your dental professional can help ensure the success of your dental work.
How to Determine Centric Relation
There are at least five ways of determining CR, according to a report in the International Journal of Dentistry and Oral Health. One common way is by manually moving the muscularly relaxed lower jaw. A skilled dentist can do this painlessly in just a few moments and record this position with a bite registration for future use. Regardless of how your dentist determines CR, it should result in the same reproducible position time after time.
Centric occlusion (CO) is the term that describes the position of the lower jaw when you bite and all your teeth come together. As explained by Spear Education, it's also known as maximum intercuspation (MIP) and differs from CR in that it is independent from the position of the jaw joint itself. Ideally, MIP would occur with the jaw in the CR position, but this is not always the case. For example, you may be able to bite with all your teeth coming together, but your lower jaw may not be seated ideally in the joint. When MIP and CR vary anatomically, it may or may not result in problems. It really depends on the individual.
Your dentist can determine if your jaw and bite are aligned properly and help you pursue any necessary treatment. Make sure you see them for checkups twice a year and report any sudden pain or jaw problems as soon as they occur.